Tramadol is a centrally acting synthetic analgesic with a dual mechanism of action attributed to the racemic form of the drug, comprised of μ-opioid activity (binding to μ-opioid receptors and monoamine (serotonin and noradrenalin) reuptake inhibition. Tramadol is an analog of the phenanthrene group of opium alkaloids, which includes morphine and codeine, and is structurally related to these opioids (Grond S and Slabotzi A. Clinical pharmacology of tramadol. Clin Pharmacokinet. 2004;43:879-923). Like codeine, there is a substitution of the methyl group on the phenol ring that imparts a relatively weak affinity for opioid receptors. (+)-Tramadol is a more potent inhibitor of serotonin uptake, while (−)-tramadol is a more potent inhibitor of norepinephrine uptake. The opioid-like activity of tramadol derives from low affinity binding of the parent compound to μ-opioid receptors and higher affinity binding of its main metabolite. Tramadol affinity to μ opioid receptors is about 10 times weaker than codeine, 60 times weaker than dextropropoxyphene, and 6,000 times weaker than morphine. The active metabolite O-desmethyltramadol (M1) possesses a higher affinity to the μ opioid receptor than tramadol and displays analgesic activity (Leppert W, 2009).
Tramadol was originally developed by the German pharmaceutical company Grünenthal GmbH in the late 1970s and is marketed globally under the trade names TRAMAL® and others outside of the United States. The approved doses of tramadol are 50 mg or 100 mg administered as a slow injection every 4-6 hours (Tramadol Core Product Label, 2008). In the U.S., tramadol is approved by the Food and Drug Administration (FDA) and marketed as an oral capsule/tablet for moderate to moderately severe pain in adults. Tramadol was first approved in the US in April 1995 under the trade name, ULTRAM® (Ortho-McNeil-Janssen Pharmaceuticals, Inc). Tramadol is also an active agent in an extended release product, Ultram® ER, and a combination product with acetaminophen, ULTRACET®. In the US, tramadol is only available as immediate release tablets or extended release tablets. Other tramadol formulations approved in several countries include tablets, capsules, effervescent powders, and suppositories (Grond and Sablotzki, 2004; Rosenberg, 2009). The approved intravenous regimen in India is an initial injection of 50 mg infusion over 2-3 min, followed by 50 mg every 10-20 minutes if necessary up to 250 mg for the first hour. Maintenance doses are 50-100 mg every 4-6 hours with a maximum dose of 600 mg daily (Tramadol, CIMS Data_India).
Postoperative pain management with tramadol has effectively utilized a variety of delivery methods, including bolus injection (IV or IM), continuous infusions and patient controlled analgesia (PCA) pumps, and various combinations of these methods (Scott and Perry, 2000; Grond and Sablotzki, 2004). The potency ratio of IV tramadol to IV morphine is approximately 1:10, while the ratio for IV fentanyl is 1:979 (Grond and Sablotzki, 2004).
The “on-demand” analgesic efficacy of tramadol was compared to morphine in the 24-hour post-operative period for 523 patients undergoing abdominal surgery (Vickers M D, Paravicini D. Comparison of tramadol with morphine for post-operative pain following abdominal surgery. Eur J. Anesthesiol. 1995;12: 265-71). Patients who reported post-operative pain received an initial dose (either tramadol 100 mg or morphine 5 mg i.v.) and, if necessary, repeat i.v. or i.m. doses of tramadol 50 mg or morphine 5 mg on demand over the first 90 minutes. Further doses up to a total of 400 mg tramadol or 40 mg morphine could then be given after 90 minutes up to 24 hours after the first dose of study medication. The primary efficacy parameter was the responder rate (no or slight pain) within the first 90 minutes of treatment. Responder rates were 72.6% for tramadol and 81.2% for morphine, which were statistically equivalent and within the predefined range of ±10%. Mean cumulative doses were 188.2 mg for the first 90 minutes and 157.1 mg for the subsequent 22.5 hours in the tramadol group and 13.9 mg and 18.4 mg, respectively in the morphine group. The main adverse events were gastrointestinal in both groups, with mild nausea, dry mouth, vomiting, dyspepsia and hiccups reported most frequently.
The analgesic effect of continuous infusion of tramadol was compared to repeated bolus administration in 135 patients undergoing abdominal surgery (Rud U, Fischer M V, Mewes R, Paravcini D., “Postoperative Analgesie mit Tramadol Kontinuierliche Infusion versus repetitive” (Postoperative analgesia with tramadol. Continuous infusion versus repetitive bolus administration), Bolusgabe Anaesthesist. 1994;43:316-321. (German)). Patients were randomized at the time of the first request for pain treatment. All patients received a loading dose of tramadol 100 mg i.v. Subsequent treatment was administered in a double-blind manner; patients in the infusion group were given a continuous infusion of tramadol 12 mg/h for 24 hours, whereas patients in the bolus group received placebo infusion. In both groups, additional bolus doses of tramadol 50 mg i.v. were given as required. Pain relief was monitored by means of a visual analog scale (VAS) up to 6 hours after surgery. The number of additional boluses and the amount of tramadol administered at 6 hours and 24 hours was also used to assess analgesic efficacy. More patients in the infusion group assessed their pain relief as excellent or good compared to the bolus group (76.5% vs 65.6%). Only a few patients complained of insufficient analgesia, with more patients in the bolus group reporting inadequate pain relief than in the infusion group (7.5% vs 4.4%). A higher percentage of patients in the bolus group required two or more boluses compared to the infusion group (59.7% vs 30.8%). After 6 hours, the average tramadol consumption was 223.5±53.7 mg in the infusion group and 176.6±63.1 mg in the bolus group (p≦0.05). After 24 hours, tramadol consumption was 449.5±66.0 mg and 201.6±83.9 mg (p≦0.001), respectively. Adverse events were reported by 25% of patients in both groups, with no significant differences and no patient terminated the trial for an adverse event. There were no significant effects on blood pressure or heart rate. The authors concluded that continuous infusion was more effective in the first 6 hours after surgery. However, excess consumption by the infusion group was statistically greater than the bolus group at both 6 hours and 24 hours post surgery.
Intermittent bolus and continuous infusion of tramadol were evaluated in a postoperative study of 35 patients undergoing major abdominal gynecologic surgery (Chrubasik J, Buzina M, Schulte-Monting J, Atanassoff P, Alon E. Intravenous tramadol for post-operative pain-comparison of intermittent dose regimens with and without maintenance infusion. Eur J. Anaesthesiol. 1992;9:23-28). The study was randomized and double-blind and used tramadol infusion 15 mg/h or saline. Additional boluses of tramadol 100 mg were given as requested. The patients in the infusion group required 60% less tramadol on demand (p<0.01) and had better pain relief (p<0.05), as assessed by VAS, than the group that received the saline infusion. Total tramadol consumption, however, was about 30% higher in the infusion group (p<0.05) and was associated with and increased incidence of minor adverse events. Tramadol was ineffective as pain relief within 2 hours of the beginning of treatment in 6% of the infusion group and 20% of the bolus group. Thus, continuous infusion was preferred to “on-demand” bolus treatment.
A meta-analysis of nine randomized, controlled trials indicated that tramadol was as effective as other opioids, including morphine, for control of postoperative pain (Scott and Perry, 2000). Pain in these patients was described as moderate to severe, with initial postoperative pain reported as >60 on a 100-point visual analog scale or as moderate or severe on a 4- or 5-point verbal response scale. The first dose of analgesia was administered when patients reported moderate to severe pain in the postoperative setting. Studies that did not adequately record baseline pain severity or response to analgesia, were not randomized or controlled or contained less than 45 patients were excluded from the meta-analysis. Tramadol, administered in a dose titrated to pain response and via either IV (intravenous) or IM (intramuscular) intermittent injection, reduced pain intensity by 46.8% to 57.6% after 4 to 6 hours compared to 69.8% for morphine and 25.6% to 51.3% for pentazocine. Efficacy of tramadol was maintained for the duration of the studies, which were ≦72 hours, and was comparable to morphine or alfentanil. However, the onset of action of tramadol was slower than morphine, as assessed by measurements approximately 3 hours after the first dose. There were no significant differences in the percentage of patients treated with tramadol or morphine and who also required rescue medication. The patient global response and physician global response were similar for tramadol and for other opioids.
Tramadol injection (IV/IM/SC) is approved and used for the management of moderate to severe acute postoperative pain in several regions, including Europe, India and Australia/New Zealand (however, this dosage form is not available in the USA). Tramadol ampoules or vials for IV, IM and SC administration and preservative-free solutions for injection by the various spinal routes (epidural, intrathecal, caudal, etc.) are available forms in these regions. Tramadol formulations approved in several countries include, tablets, capsules, effervescent powders, and suppositories (Grond and Sablotzki, 2004; Rosenberg, 2009).
There is extensive data demonstrating that tramadol use is not associated with the classical opioid side effects seen with more potent opioids. There are numerous reports of the safety and efficacy of tramadol (Lee et al., 1993; Scott and Perry, 2000; Grond and Sablotzki, 2004). The most common adverse events of tramadol administration are nausea, dizziness, headache, somnolence, sweating, fatigue, constipation, dry mouth and vomiting. However, tramadol use, particularly with high doses, has been associated with seizures, and the risk of seizures is increased in the presence of drugs that reduce seizure threshold, head trauma or prior history of seizures. Patients undergoing surgery, for example, total knee arthroplasty (TKA) and total hip arthroplasty (THA), typically demonstrate a need for short-term analgesia, which is critical for earlier mobilization and rehabilitation. In this setting, assuring adequate pain relief without providing extensive medical oversight required for some methods of treatment (such as neuraxial anesthesia) and prevention of effects such as opiate-induced respiratory depression and dependency would be highly beneficial (Sinatra et al., 2002).
The goal of post-surgical pain management is twofold: i) to provide a quick onset of analgesic or pain relief and ii) to reduce or modulate the quality and intensity of pain a patient experiences in the post-surgical period. While current treatments for management of post-surgical acute pain are useful, there is a need for improved methods for treating post-surgical acute pain.